It is desirable for electrical devices, particularly electrical devices that include an electric motor, to be equipped with a thermal protector that will stop operation of the device in the event that the operating temperature of the device exceeds a predetermined maximum operating temperature. Stopping the device is desirable because it can prevent mechanical damage from occurring and frequently can prevent an electrical motor from operating at unacceptably high temperatures. A number of devices that perform this general function are know in the prior art.
U.S. Pat. No. 6,553,611 discloses a thermal protector device for utilization in conjunction with vacuum cleaners. In vacuum cleaners, overheating of the motor typically results from a blocked or plugged filter, or from one or more objects interfering with the operation of the rotating brush or floor element. U.S. Pat. No. 6,553,611 discloses a vacuum cleaner comprising: a lower base unit; an upper enclosure being pivotable with respect to the lower base unit; a motor disposed within the lower base unit; a power cord having a first end affixed to at least one of said upper portion and said base unit, and a second end adapted for connecting to an electrical power source; electrical conductors extending between said first end of said power cord and said motor, said electrical conductors defining an electrical power circuit to said motor; and a thermal cutoff assembly including a temperature sensor disposed proximate to said motor for measuring the temperature of said motor, said thermal cutoff assembly further including a switching element in electrical association with said electrical conductors, wherein upon the temperature sensor sensing a temperature greater than a predetermined temperature setpoint, said switching element opens said electrical power circuit; wherein once said switching element has opened said electrical power circuit, said switching element closes said electrical power circuit only upon the temperature sensor sensing a temperature less than the predetermined temperature setpoint and after said thermal cutoff assembly has been disconnected from said electrical power source.
U.S. Pat. No. 4,703,298 discloses a thermostat that includes ceramic mounting pins that are made of ceramic PTC material. The thermostat is a non-enclosed device and is, therefore, subject to atmospheric conditions. Two metal contact carriers are fastened onto parallel pins. Carriers are selectively moveable relative to each other on pins, so that the thermostat can be adjusted for a specific circuit opening temperature. The thermostat is required to be non-enclosed to permit the carriers to move with respect to the pins.
In use, when the bimetal moves to the open position, the temperature is such that the resistance of the PTC material is substantial, so that the current which now flows through the pins generates sufficient heat to keep open the circuit between carriers. This current flow, together with the selective resistivity of the pins at this temperature, is sufficient to maintain the bimetal above its reset temperature even though the ambient temperature being monitored by the thermostat may return to its original or normal level. The thermostat thus remains open until it is allowed to reset because it has been disconnected from the circuit, whereby the heat source is removed and the bimetal is permitted to snap to the closed contact position because it is at a temperature below the reset temperature.
U.S. Pat. No. 3,525,914 discloses a thermo switch that includes a ceramic PTC heat resistor that is mounted between the inner surfaces of the bimetallic strips. Similarly, U.S. Pat. No. 5,309,131 discloses utilizing a PTC resistor mounted between a fixed contact and a moveable contact. In all these disclosures, current flows through the PTC resistor regardless of whether the switch is open or closed. The resistance of the PTC resistor increases with increasing ambient temperature. The switches of U.S. Pat. No. 4,703,298, U.S. Pat. No. 3,525,914, and U.S. Pat. No. 5,309,131 open when the ambient temperature is above the reset temperature. The PTC resistor in each is designed to generate a sufficient amount of heat so that the temperature is above the reset temperature to maintain the switch open. Thus, the switch of either U.S. Pat. No. 4,703,298, U.S. Pat. No. 3,525,914, and U.S. Pat. No. 5,309,131 remains open until the load current is removed.
A PTC resistor is designed to have a relatively low resistance when the ambient temperature is below a threshold value. Thus, when the switch is closed, current flows through the PTC resistor, but because the resistance is relatively low, an insufficient amount of heat is generated to increase the temperature at the thermostat above the actuation level. Only when the ambient temperature rises above a predetermined point is the resistance of the PTC sufficient to generate significant heat.
For many uses it is desirable for the thermostat to be enclosed, sealed from the local ambient conditions to ensure the efficient operation of the thermostat. Sealed thermostats can be calibrated or adjusted for a specific circuit opening temperature by distorting the case at a predetermined location as is well known in the art (see, for example, U.S. Pat. No. 3,443,259 and U.S. Pat. No. 3,223,808).
U.S. Pat. No. 5,936,510 discloses a thermostat comprising: (1) a case having a sealed interior and an exterior; (2) a contact extending from said sealed interior to said exterior, said contact being fixed with respect to said casing and having an interior contact position; (3) a first blade extending from said sealed interior to said exterior, said first blade having an interior end, said interior end of said first blade moving between a first position where said interior end of first blade abuts said interior contact position of said contact and a second position where said interior end of said first blade is spaced from said interior contact position of said contact; (4) a separator disposed between said contact and said first blade in said interior of said case, said separator being made of an insulating material, said separator having a first side facing said contact and a second side facing said first blade; (5) a first conductive contact pad disposed on said first side of said separator; (6) a second conductive contact pad disposed on said second side of said separator; and (7) a resistor disposed within said separator being electrically connected between said first conductive contact pad and said second conductive contact pad, said resistor having a sufficient resistance so that when said interior end of said first blade moves from said first position to said second position a sufficient amount of heat is generated by said resistor to maintain said interior end of said first blade in said second position until a load current being applied to said contact and said first blade is removed.
U.S. Pat. No. 6,020,807 discloses a thermostat comprising: (1) a case made of conductive material, said case having an exterior; (2) a cover plate made of a conductive material, said cover plate being connected to said case to define a sealed interior, said cover plate having a contact projecting into said sealed interior, said contact being fixed with respect to said case and having an interior contact position; (3) an insulator sheet disposed between said case and said cover plate; (4) a blade disposed in said sealed interior, said blade having a second end, said second end of said blade moving between a first position where said second end of said blade abuts said contact position of said contact and a second position where said second end of said blade is spaced from said interior contact position of said contact; and (5) a resistor mounted on a first end of said blade, said resistor being disposed between said cover plate and said blade in said sealed interior of said case, said resistor having a first side facing and abutting said cover plate and a second side facing and abutting said blade so that said resistor is electrically connected between said cover plate and said blade, said resistor having a sufficient resistance so that when said second end of said blade moves from said first position to said second position a sufficient amount of heat is generated by current flowing through said resistor to maintain said second end of said blade in said second position until a load current applied to said cover plate and said blade is removed.
U.S. Pat. No. 5,309,131 discloses a thermal switch having two connecting parts fixed to a support part. One connecting part carries a fixed contact and the other connecting part is connected through a high resistance to a movable contact switchable by a bimetallic element. The support part is a ceramic part forming the high resistance from a PTC material and the connecting parts are almost completely frictionally engage support part.
U.S. Pat. No. 4,847,587 discloses a bimetal thermoswitch consisting of an electrically insulating, flat carrier, at least two electric terminals which are secured to the carrier and are respectively connected to a fixed contact, which is mounted on the carrier, and to a contact spring, which is secured at one end to the carrier and at its other end carries a movable contact element, which cooperates with the fixed contact. The carrier consists of an alumina ceramic slab.
U.S. Pat. No. 4,862,133 discloses a small-size thermal switch having a base formed of an electrically conductive plastic. The thermal switch is arranged such that, after a bimetallic element deflects in a reverse direction and opens the circuit of the thermal switch, the bimetallic element is self-held in such a reversely deflected state and the self-held state of the bimetallic element is maintained until an electrical power source is switched off. U.S. Pat. No. 4,862,133 further provides a thermal switch in which one surface of its movable-contact leaf spring is maintained in contact with the surface of its bimetallic element which is formed of a metal having the lower coefficient of expansion. Since the bimetallic element is sandwiched between the base and the movable contact, the stability of the bimetallic element in a recess of the base is improved.
There is currently a need for a thermal protector which is capable of continuous operation at 20 amps and preferably at 25 amps or greater at 120 volts of alternating current having a self hold feature which will not allow current to be restored on cooling without breaking the supply of electrical current by unplugging the device from its electrical supply or switching off the electrical supply to the device. There is also a need for such a thermal protector that will also break the hold of current on detection of unacceptably high currents. There is a further need for a self hold thermal protector which is capable of at least 60 cycles of operation at a peak amperage of at least 90 amps at 120 volts of alternating current. However, self hold thermal protectors that are available today are not capable of this combination of needed characteristics.